The friction and wear of zirconia sliding on zirconia at low speed (1 mm s−1) and moderate load (9.8 N) were studied with a pin-on-disk machine. Two materials were investigated, a brittle (2.5 MPa m), cubic phase doped with 5.5 mass% yttria and a tough (11.6 MPa m), tetragonal phase stabilized with 3 mass% yttria. Sliding occurred in dry nitrogen, where mechanical effects alone are expected, in laboratory air (relative humidity, 50% ± 10%), in water, in pure hexadecane and in hexadecane containing 0.5 wt.% stearic acid. Friction coefficients ƒ depend on the environment and somewhat on toughness (ƒ = 0.1 in hexadecane containing stearic acid, 0.15 in pure hexadecane, 0.3–0.6 in air, 0.7 in water and 1.0 in dry N2). All wear rates decrease with increasing sliding distance. They are in the range 10−5-3 × 10−4mm3N−1m−1 for the tough material. Environmental effects are strong; they are compatible with stress corrosion cracking and not with the tribochemistry that governs environmental effects in non-oxide ceramics such as Si34. For the tough zirconia, the wear rate is lowest in dry nitrogen. For the other environments, it increases in the following order: hexadecane, hexadecane containing stearic acid, air, water. With the brittle material, wear is lowest in hexadecane and highest in air. The results are analyzed with the help of a model that relates the wear rate to the local contact stresses.